The long-term goal of this project is to develop a safe and effective vaccine against West Nile virus infection. West Nile virus (WNV) has recently attracted considerable attention because of the outbreak in New York City in the summer of 1999. The virus has since been isolated from mosquitos and birds in Connecticut. suggesting that it may have established itself, and the possibility of future outbreaks can therefore not be excluded. In fact, a recent survey of 2,300 mosquitoes identified several samples that were positive in a PCR assay designed to detect WNV RNA. This suggest that the virus may have survived the winter. Control of WNV spread is severely hampered by the fact that a vaccine is not available. On the other hand, the paradigm for flavivirus vaccines is the YFV-17D vaccine strain. The YFV-17D vaccine strain is a live, attenuated virus vaccine, provides effective immunity against pathogenic YFV strains and has an excellent safety record. Here, we propose to construct a chimeric YF/WN virus and test this as a candidate vaccine against WNV in both mouse and monkey challenge models. The chimeric virus would be expected to induce both neutralizing antibodies and T cell responses against the WNV prM/E proteins. The main goal of this project to generate a putative WNV vaccine that is optimally balanced in terms of attenuation and immunogenicity. The emphasis of this study will therefore be on these two aspects. The Specific Aims that we want to pursue are as follows: (i) To construct chimeric, recombinant viruses based on the YFV-17D genome, that express West Nile virus pre-membrane (prM) and envelope (E) proteins. (ii) To test the attenuation and immunogenicity of the chimeric viruses and their ability to confer protective immunity against West Nile virus infection in immune and monkey models (iii) To test whether pre-existing immunity against YFV-17D will interfere with the immunogenicity and efficacy of a subsequent YF/WN vaccination and vice versa.